Bridge amplifier circuit



y 5, 1953 R. w; BORDEWIECK 2,637,786

BRIDGE AMPLIFIER CIRCUIT Filed June 22, 1950 IN VEN TOR;

Mme

Patented May 1 953 BRIDGE AMPLIFIERUI RCUIT Robert W. Bordcwieck,Southboro, Mass., assignorto MoorelElectronicLaboratories lncorporated,.Worcester, Mass a; corporation of; Massach'ursetts Application June 22,1950,,ScrialNor-1fi9fl2di 2 Claims. 1

This invention relates to improvements in a bridge amplifier circuit,wherein a Very low-impedance balanced-output is provided fora very highgain bridgeamplifien- This makes possible the use of a simple bridgeamplifier with the advantages of high stability and constant gain aswellas absence of diflicultie's caused bysh'iftin tube characteristics,vibration, thermal effects, etc.

These objects are attained by methodsdescribed in the accompanyingdrawing in which the figure shows a schematic electric diagram of oneform of the invention.

Referring to the figure, numeral is a source of filament supply voltageand high voltage D. C. for the bridge amplifier and low impedancebalanced output circuit. Numerals I2 and I4 are terminals connected tothe heaters X, X of the cathode follower tubes 10 and 80. Numerals l6and I8 are terminals connected to the heaters Y, Y of the bridgeamplifier tubes 40 and 50. High voltage terminal 22 is connected bylines 24 and 25 to plates 16 and 86 of cathode follower tubes 10 and 80respectively, and through ballast resistor 2B and line 28 to voltageregulator tube 34 and thence through lines 36, 38, and 90 to a secondvoltage regulator tube 92, and thence to the low voltage return by line98.

The source of regulated voltage at point 29 is connected by line 30 tothe plate load resistors 41 and 51 respectively of bridge amplifiertubes 40 and 50 respectively; and the source of regulated voltage atpoint 29 is connected by line 30 through screen dropping resistors 45and 55 to the screens 46 and 56 of bridge amplifier tube 49 and 50respectively. Suppressors 48 and 58 are connected to the cathodes 42 and52 respectively and thence by common line 4| to cathode resistor 60 andto a balancing network consisting of potentiometer GI and resistors 62and 53 connected in parallel with potentiometer 6| and thence throughlines 94, 96, and 98 to the low voltage return point 20.

The grid 54 of bridge tube 50 is connected by grid resistor 64 to thejunction point between resistors 62 and 63. The grid 44 of tube 40 isconnected through grid resistor W0 and line I03 to the'slider I02 ofpotentiometer 6|. The high side of the input to the amplifier isdirectly connected from terminal I04 to grid 44 and the low side of theinput connects from terminal I00 to the slider I02 which may be used asa ground reference by strapping together points I08A and I09 byconnecting link I01, or these may be left floating by removal of linkI01.

The source of regulated voltage at point 29 is also connected through ableeder biasing resistor 2 32 .to: the-iunctionpoint ofresistors 0 l..,and B2,.toprovide a-rsmallamount of fixed bias used in; conjunction withthe self-bias developed across resistor 6.0 and the biasing network;consisting of resistors iii, 62 and 63.- Plate 49.is connectedby line,49A to-. grid84 of cathode :follower tube 80. Plate 59isconnected byline 59A to grid 14 of cathode, follower-tube .10.. CathodelZ of cathodefollower'tube. 10 .is connected to point 10A which will hereinafter becalled output terminal A, and. also through cathode load resistor "IIand line 38 to the voltage regulator tube 92, and thence to the B minusreturn through line 98. Cathode 82 is connected to terminal A which willhereinafter be called output terminal B and through cathode resistor 8|to voltage regulator tube 92.

An electro-mechanical device I I0 is shown, this device representing anyof the various types of indicators, recorders, relays, solenoids, etc,which can be used to advantage in the output of the circuit of thefigure. Since it is the basic circuit which is here presented and not aspecific application, it is not desired to be limited to the use of arelay, recorder, or indicator, but it is to be understood that thepresent circuit can be used to advantage with any electrical ouputdevice.

The operation of this circuit is as follows:

With no signal impressed on input I04, the slider I02 is moved up ordown to vary the initial bias on the grid 44 of tube 40 either above orbelow that of grid 54, so as to effect a perfect balance between thevoltages at the plates 49 and 59 of the bridge amplifier tubes, or if itis desired to initially offset the balance at the plates of these tubesto a predetermined amount of either polarity, this may be done. Thevoltages at the plates 49 and 59 are directly coupled to the grids 84and 14 respectively of cathode follower tubes 80 and 10 which inthemselves would provide an appreciably reduced impedance. That is,several megohms could be used for plate load resistors 41 and 51 andcathode resistors 7i and BI could be made in the order of 10,000 ohms.However, by the novel addition of a voltage regulator tube between thecathode follower resistors 8| and H and the B minus return, it ispossible to use resistors on the order of several hundred ohms in thecathodes of the cathode follower tubes. By this development it ispossible to produce an extremely low output impedance from the outputcathode follower stage and still maintain all of the advantages that abalanced bridge amplifier circuit can possess plus additional advantagesarising from the much higher plate load resistors which can be used ifthe bridge amplifier tube does not have to drive the load directly.

Having thus described my invention and the advantages thereof, I do notwish to be limited to the details herein disclosed otherwise than as setforth in the claims, but What I claim is:

1. A bridge amplifier comprising a source of electrical energy, a pairof amplifier tubes each having at least a plate, a cathode, and acontrol grid, a pair of cathode follower output tubes each having atleast a plate, a cathode, and a control grid, and a voltage regulatortube, the said amplifier tube plates being connected through separateplate resistors to the positive terminal of said source of electricalenergy, the said amplifier tube cathodes being connected togetherthrough a common self-biasing network to the negative terminal of saidsource of electrical energy, the cathode follower tube plates beingconnected to the positive terminal of said source of electrical energy,the said cathode follower tube control grids being connected separatelyto the plates of the said amplifier tubes, the cathode follower tubecathodes being connected through separate cathode resistors to the anodeof the said voltage regulator tube, and the cathode of the said voltageregulator tube being connected to the negative terminal of the saidsource of electrical energy, the amplifier output being taken betweenthe cathodes of the said cathode follower tubes, the said voltageregulator tube and the said cathode resistors being effective incombination to provide the amplifier with very low impedance output.

2. The circuit of the preceding claim wherein the said self-biasingnetwork comprises two resistance branches, the first said branch beingtapped and the tap connected to the control grid of one of saidamplifier tubes, and the second said branch comprising a potentiometerhaving its slider connected through the amplifier input circuit to thecontrol grid of the other said amplifier tube, said network comprisingmeans whereby the bias of one of said control grids may be adjustedselectively above, equal to, or below the bias of the other said controlgrid.

ROBERT W. BORDEWIECK.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,286,894 Browne et a1 June 16, 1942 2,490,167 Storm Dec. 6,1949 OTHER REFERENCES Text Book-Vacuum Tube Amplifiers-Valley, Jr., andWallman-Radiation Laboratory Series-1948 edition-McGraw-Hill Book Co.,publisher, page 476, p. 487.

